CA1115509A - Carbon coating for a starting member used in producing optical waveguide - Google Patents

Abstract

Dominick-Scherer 2-1 D-7309 (CO-76) CARBON COATING FOR A STARTING MEMBER USED
IN PRODUCING OPTICAL WAVEGUIDE

Abstract Of The Invention In making a preform for an optical waveguide, the bait rod is coated with carbon. After the preform is built up on the bait rod, it is easily removed. The carbon coating produces a smooth improved inner surface in the preform, thereby eliminating flaws which otherwise may be present when the preform is drawn into a waveguide.

Description

~ 5~ 080g77 D-7309 (C0-76) Back round Of The Invention This invention relates to optical waveguides and more particularly to an improved process for making the pre-form for an optical waveguide.
Waveguides us'ed in optical communications syst~ms are herein referred to as "opkical waveguides" and are normally constructed from a transparent dielectric material such as glass or plastic.
U. S. Patent 3,775,075 - Xeck and Maurer describes a process of making optical wa~eguides wherein a glass ' coating is deposited by flame oxidation* upon a rotating glass cylinder. Thereafter the formed structure is heated and'draw~ into a waveguide with the glass cylinder forming the core and the coated material forming the cladding of the optical waveguide. The glass cylinder upon which the cladding material is coated is referred to as a preform.
U. S. Patent 3,823,9~5 - Carpenter describes one technique of making a preform. In this technique, glass is deposited by flame outside vapor phase oxidation (oVP0) on a rotating starting member which is referred to as a bait rod. After the core and cladding layers are deposited, the bait rod is removed. Then the structure is heated and drawn.
During the drawing operation, the hollow core of the preform collapses, thereby forming a consolidated core with a cladding.
The removal of the bait rod from the preform must be carefully carried out because imperfections on the inside surface or the preform may cause flaws in the finished waveguide. Imperfections on the inner surface of the preform *Now called outside vapor phase oxidation (oVP0); the reaction is known to be oxidation rather than hydrolysis.

~ $5~ 08~977 D-73D3 (C0-76) propagate cracking and breaking. Any imperfection in the center hole may cause bubbles to form in the waveguide which is ultimately produced causing high attenuation and rejection. As described in the aforementioned Carpenter patent, the bait rod is removed by grinding it out by means of a diamond reamer. That patent also mentions core drilling and hydrofluoric acid etching for removlng the bait rod.
The rough surfaces left by drilling may be smoothed by laser milling, mechanically polishing the inside surface, fire polishing the inside surface and/or washing the tube in hydro-fluoric acid. ~11 of these operations are time-consuming.
More recently, the preform has been m~de by iirst depositing a soft layer of soot on the bait rod and thereafter building up the preform with hard soot. The layer of soft soot facilitates removal of the bait rod The 50ft layer shears, leaving a flaky pitted center hole in the preform.
After sint ring the preform to consolidate the glass, the preform still contains these flaws. A lengthy hydro~luoric acid treatment is necessary for elimination of these imper-iactions. Damage occurs because the preform adheres so strongly to the surface of the bait rod. When the rod is removed, the preform shears rather than releasing from the bait rod surface.
~. S. Patent 3,933,453 - Burke et al, describes an improved ma~drel including a tubular member formed o refractory metal. U. S. Patent 3,806,570 - Flamenbaum et al descri~es the us~ of a fused carbon mandrel for the bait rod. ~owever, neither of these mandrels can be removed from the preform without causing blemishes which require smoothing Imperfections in the center hole of the preorm are a particular problem in making gradient index waveguides 5~3 D-7309 (C0-76~

such as described in the afoxementioned Carpenter patent.
In order to get a high numerical aperture waveguide, it is necessary to introduce a sharp radial change in composition of the preform. Because of the changing thermal expansion coefficient of this composition, high stress is present. In such waveguides, it is particularly important to eliminate Lmperfections in the center hole of the preform. Any imper-fection may lead to fracture in a high stress waveguide.

Summar Of The Invention Y _ _ ~

1~ In accordance with this invention, imperfections in the center hole of a preform for an optical waveguide are reduced by coating the starting member with carbon before ~epositing the preform thereon. Thereaft~r the starting member can be removed with little detrimental effect on the preform. Separation occurs at the interface between the carbon and the starting member or by shearing of the carbon.
There is no shearing of the preform. The preform has a very smooth, undisturbed centPr hole.
~he preform is then conventionally heated to con-solidate the glass~ During consolidation, the carbon oxidizes, thereby completely disLntegrating it. After consolidation, the preform has a smooth clean inner surface. Whereas lengthy hydrofluoric acid treatments were previously necessary to ~mooth the inner hole of the pre~orm, this is no longer necessary. The inner hole can be allowed to close during the consolidation procedure, whereas in the prior art, the inner hole was not closed until drawing. Because the inner hole is closed during consolidation, it is possible to draw the waveguide at higher speeds than were previously possible.
It is an object of the present invention to provide improved optical waveguide preforms with reduced imperfections on the inside surface thereof.
It is another object of the present invention to produce high numerical aperture, high stress waveguides from preforms with relatively smooth inner surfaces.
It is another object of the present invention to eliminate the etching step in the production o~ optical waveguides.
It is further ob~ect of the present invention to increase the drawing speed in the production of waveguides.
The foregoing and other objects, features and advantages of the invention will be better understood from the following more detailed description and appended claims.
Thus the present invention provides a method of making a preform for an optical waveguide cornprising: providing a substantially cylindrical starting member; applying a coating of substantially pure particulate carbon to the outside peripheral surface of said cylindrical starting member; applying particulate material tv said starting member to form an adherent coating of glass; removing said starting member from said glass to cause separation between said carbon layer and the outside pexipheral surface of said starting member or shearing within the carbon layer to produce a preform; and heating said preform to a temperature ~hich disintegrates said carbon by oxidation and which consolidates said preform.
In one aspect the invention provides such a method wherein the step of applying particulate material to said starting member includes depositing a coating of glass soot on the outside peripheral surface of said starting member by flame outside vapor phase oxidation. In a preferred embodi-ment this may be effected by entraining a gaseous material predetermined amounts of each of a plurality of constituents in vapor form; oxidi~ing said vapors in a ~lame to form a soot having a predetermined composition and applying said soot to the exterior peripheral surface of said member. In another preferred aspect the invention provides such a method further comprising radially varying the composition of the soot deposited on said starting member to produce a preform having a radially varying index of refraction.
In another embodiment the present invention provides such a method as set out broadly above further comprising applying a coating of cladding glass having an index of refraction less than that of said first named coating, the composition of said cladding glass being compatible with that of said first named coating.
The present divisional specification is directed to the provision of an apparatus for making preforms or optical waveguides comprisingD a substantially cylindrical starting member having a smooth outside peripheral surface;
a coating of particulate carbon without impurities on the outside periphery of said starting member, said carbon being present in an amount which is completely disintegrated during heating to the consolidation temperature of glassmaking particulate material; and means for applying said partic~late material to said starting m~mber to form an adherent coating of glass.
Preferably said starting member is tapered. The starting member may be an aluminum oxide mandril.

S~

In another preferred embodiment the means for applying particulate material to said starting member includes:
means for entraining predetermined amounts of each of a plurality of constituents in a vapor, and a burner for oxidizing said vapors in a flame to form a soot having a predetermined composition.

Description Of The Drawings Fig. 1 depicts the deposition of the carbon coating;
Fig. 2 depicts the step of applying glass to the preform by flame hydrolysis;
Fig. 3 depicts heating to consolidate, burn the carbon and collapse the center hole; and Fig. 4 depicts the step of drawing the preform into a waveguide.

Description Of The Preferred Embodiment ; Referring to Fig. 1, a substantially cylindrical starting member 10 is continuously rotated in an acetylene _, ~

"- 0~1077 D-7309 (C0-76) flame 11 to produce a carbon coating 12 on the starting member. For example, the starting member may be a tapered alumina bait rod tapering from .630 to .570 centimeters diameter along a 63 centimeter length. Acetylene burner 13 burns acetylene in air (no oxygen gas supply) to produce carhon soo~. Bai~ rod 10 is mounted in a lathe which rotates the rod and tra~erses it past the flame 11. It has been found that the thickness of the coating is not critical but 1-2 mills thickness produces satisfactory results.
Ater the bait rod is thoroughly coated with carbon, glass soot:is deposited by flame hydrolysis as depicted in Fig. 2~ The procedure described in the afore-m~ntioned Carpenter patent may be used to produce a gradient index waveguide. Alternatively, step index waveguides with a central core with one index of refraction and a cladding layer of a lvwer index of refraction may be produced. An adherent layer of glass soot 14 is built up on the bait rod 10 as it is rotated and translated in the hydrolysis flame 15. Both the preform 14 and the carbon coating 12 can be removed from the bait rod with no detrimental effect.
Preform removal rPsults from separation at the carbon-bait rod interface or from shearing of the carbon and not from shearing of the preorm. This allows protection of the inner surface of the preform and easy removal of the bait rod.
Examination of the preform shows a very smooth, undisturhed center hole.
The preform is then sintered in the usual manner as i5 depicted in Fig. 3. The carbon completely disintegrates by oxidation at about 800~ C. which is approximately 5~0 lower than normal consolidation temperature. Inspection after consolidation shows a smooth clean inner surface that requires ~ 5~ 0977 D-7309 (C0-76) little, if any, hydrofluoric acid treatment. In many instances, the etching step may be eliminated completely, i~ which case the consolidation step can be carried out in such a way as to close the center hole. This allows faster, more economical operation of the draw facility which is depicted in Fig. 4.
Furthermore, closure of the center hole during consolidation permits the making of preforms with high internal stress (high radial change in composition and refrac-tive index) because the elimination of the hole~ during consoli-dation removes one of the prominent sources of fractures in waveguides. This commonly occurs because of flaws on the interior surface of the consolidated ~i.e. sintered) preform. Also, the invention has particular advantages in making high numerical aperture guides which require high dopant levels and consequently greater stress caused by steep thenmal expansion coefficient changes, because, the smooth inner surface on the consolidat~d preform made in accordance with this invention can tolerate hish stress.
Another advantage of the invention is the elimina-tion of etching away approximately 4% of the weight of the preform as is routinely done in present practice wh re soft soot is initially deposited on the bait rod to facilitate bait removal and is later etched away.

The carbon coating can be applied by several other methods including dipping the rod in a carbon slurry or dipping the bait rod in wax and then charring it. The carbon coating must burn off during consolidation and the coating must not impart impurities to the preform. The following ss~

D-7309 ~C0-76) exampl~s demonstrate the feasibility of this invention.

E X A M P L E ~ 1 A 33 wt. % AguadagR E, Aqueous graphite dispersion, mixture was made in distilled water. Final weight percent graphite was 13%. This mixture was rolled for several hours and then poured into a PyrexR tube approximately 30 inches long and 3/4 n in diameter.
A clean, tapered alumina r~d about 63 cm. long and tapering from .630 to .570 cm. diameter was carefully dipped in the mixture and removed. The surface of the adherent layer of graphite was inspected for flaws - lumps, inclusions, uneven areas - and allowed to air dry at room temperature several hours.
The prepared bait was next secured in a lathe normally used for preform production. Standard laydown procedures were used for the productior of a parabolic blank except the soft preliminary soot laydown was omitted. The GeO2 content was varied parabolically from 15 to O wt. %
across the core, while B203 content was raised from 2 to 12 wt. %.
The soot was deposited by a burner having an inner and outer shield, both of which supply oxygen, and inner burners for tAe other gases. A gradient index guide was produced in which the BC13 was increased and the GeC14 was decreased as a ramp function of time. The settings used were as follows: , Outer Shield 4.0 L/min.
Inner Shield 4.5 L/min.

~ S6'~ ~80977 D-7309 (C0-76) Premix Gas 6.0 L/min.
Premix Oxygen 5.0 ~/min.
By-pass 2 0 5 h/min.
SiC14 8.0 ~m/min.
BC13 Ramp Program .097 gm/min. - 1.17 gm/min.
GeC14 Ramp Program .8 gm/min. - O gm/min.

Traverse Speed ~ 98cm/min.
Rotation Speed ~ 300 RPM
Burner to bait distance 14 cm 1~ The center hole of this preform after bait removal was excellent, no major f}aws were noticed and it appeared very smooth and relatively undisturbed. The preform slipped from the bait rod far more easily than when the carbon coating was not used.
Next, the preform was consolidated at 1320nC in Helium with a feed rate into the furnace of .l inch/min. After consolidation the blank was allowed to ~:ool and the inner surface examined. A significant improvement of inner hole surface was noted. The usual pits and scratch marks were absent. The preform was etched and then drawn into waveguides of 125 um diameterO These waveguides had the following properties:

Fiber Reel # Attenuation NA Band Width 1 9.2 d3/km .146 850 Megahertz ~ 4.8 .146 560 3 4.0 .150 560 4 3.8 .156 560 4.5 .150 540 4.8 .150 500 7 5,3 ~150 4gO
8 5.1 .150 480 9 5.6 .150 420 ~ g_ ~ S~ 080977 D-7309 (C0-76) These results show that the carbon does not adversely affect the attenuation of the waveguide.

E X A M P L E # 2 An alumina bait rod of the type described above S was coated with carbon soot, as follows: the rod was mounted in a lathe and rotated at 300 RPM; a flame consisting of acetylene burning in air was held under the rod so that the carbon soot produced by the 1ame was deposited on the rod;
by moving the torch along the length of the rod, a uniform layer of carbon was deposited.
A preform was deposited on the coated bait rod, consolidated and drawn into fiber, all as described in example #1. The measured properties of the resulting optical waveguide are listed below: -- 15 ~iber Reel # Attenuation Nh Band W_ th 1 4.3 dB/km ~136 ~90 Megahertz

2 5.2 .129 *

3 5.7 .139' 770 While a particular embodiment of the invention has been shown and described, various modifications are within the true spirit and scope of the invention. The appended claims are intended to cover all such modifications.

*Too high to be measured, i.e. at or near the theoretical limit.

Claims (4)

Div.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Apparatus for making preforms for optical waveguides comprising:
a substantially cylindrical starting member having a smooth outside peripheral surface;
a coating of particulate carbon without impurities on the outside periphery of said starting member, said carbon being present in an amount which is completely disintegrated during heating to the consolidation temperature of glassmaking particulate material; and means for applying said particulate material to said starting member to form an adherent coating of glass.

2. The apparatus recited in claim 1 wherein said starting member is tapered.

3. The apparatus recited in claim 1 wherein said starting member is an aluminum oxide mandril.

4. The apparatus recited in claim 1 wherein the means for applying particulate material to said starting member includes:
means for entraining predetermined amounts of each of a plurality of constituents in a vapor; and a burner for oxidizing said vapors in a flame to form a soot having a predetermined composition.